The objective of this article is to firstly analyze the forms of measurement of corporate diversification through a comparative analysis of existing measures and secondly, to understand the motivations of Spanish manufacturing companies to perform relational diversification. To this end, a preliminary literature review on forms of measurement was conducted and then, two theories were studied: The Theory of Industrial Economy and the Theory of Resources and Capabilities. These theories provided a solid foundation for explaining why companies diversify to related markets. To test the model, data was obtained from a database that contains information about the Survey of Managerial Strategies (ESEE), for the period between 1991 and 2002. Binomial logistic regression was performed on a complete set of observations of diversified manufacturing companies. The results of the study showed that industrial concentration, industrial profitability, and intangible assets are factors that determine company diversification toward related markets.